This invention pertains to thermoformable coatings applied to substrates, and more particularly to typically two-stage heat curable coatings applied to thermoformable substrates such as plastics. The coating is partially cured in a first stage to form a thermoformable coating layer adhered to the substrate and heat-cured in a second stage to additionally cure the coating and provide a hard surface coating on an article having a desired configuration.
More specifically, in a first embodiment this invention relates to fluorinated polyoxetane-polyester polymers containing polyoxetane derived from polymerizing oxetane monomers having partially or fully fluorinated pendent side chains. Poly-oxetane-polyester polymers have many of the desirable properties of fluorinated polymers and the ease of processability of polyesters. The desirable properties of the fluorinated oxetane polymers are due to the fluorinated side chains and their tendency to be disproportionately present at the air exposed surface when cured. The fluorinated polyoxetane-polyester polymers are cured with an alkyl-modified melamine formaldehyde crosslinker comprising an alkyl etherified melamine formaldehyde resin.
In a second embodiment, a coating can be made with a polyoxetane-free polyester and cured in a multistage process. Specifically, the coating comprises a polyester which is cured using an alkyl-modified melamine formaldehyde crosslinking agent such as alkyl-etherified melamine formaldehyde. These polyoxetane-free compositions have a good balance of properties and are suitable for coating thermoformable substrates.
Thermoformable sheet substrates such as poly(vinyl chloride) (PVC) are used with polymeric coated surfaces comprising crosslinked polymers to provide hard surfaces exhibiting considerably increased durability. In the past, coating integrity and hardness were achieved with various types of crosslinked polymers forming a thermoset polymer network, which worked well with flat surfaces but which had limited extensibility and elasticity and, consequently, could not be thermoformed into contours and configurations without integrity failure (e.g., cracking). Hence, providing a crosslinked coating system for coating thermoformable sheet substrates (e.g., PVC) with sufficient coating integrity and extensibility to adhere while exhibiting sufficient flexibility to maintain coating integrity during subsequent thermoforming process remains desirable.
Melamine-crosslinked polyester coatings are used in low and high pressure laminates having flat surfaces. High pressure laminates typically consist of a multi-layer paper impregnated with melamine-based coatings, where the impregnated laminate is cured at relatively high temperature and pressure to produce a finished article having a hard and durable surface. Examples of this approach include a plasticized PVC layer having a surface coating that includes (i) a reactive carboxyl-functional polyester crosslinked with alkylated benzoguanamine, urea or melamine formaldehyde resin or (ii) a water-based polyester crosslinked with an acid-catalyzed amino resin.
Oxetane polymers with pendent fluorinated chains have low surface energy, high hydrophobicity, oleophobicity and a low coefficient of friction. Various oxetane monomers and polymers are described in, e.g., U.S. Pat. Nos. 5,650,483; 5,468,841; 5,654,450; 5,663,289; 5,668,250; and 5,668,251, and the interested reader is directed to these for more information.